With recent densification of electronic devices, a printed circuit board used for them has been more multi-layered and a multi-layered flexible printed circuit board has been frequently used. Such a printed circuit board is a rigid-flex printed circuit board as a composite substrate consisting of a flexible printed circuit board and a rigid printed circuit board, which has expanded in application.
According to a proposed method in the related art, a multilayered flexible printed circuit board or rigid-flex printed circuit board is prepared by alternately stacking a single-sided circuit board and an adhesive layer multiple times to form a stack, forming an interlayer-connecting through-hole in the stack, plating the interlayer-connecting through-hole and processing the outermost layer to form, for example, a circuit and the like, or by forming a hole which does not penetrate a copper foil in the insulator side of a single-sided circuit board, forming a conductor post from a metal or alloy, conducting coating of the whole surface, pressing the adhesive layer and the circuit board and repeating the process as necessary for multilayering (for example, Japanese Laid-open Patent Publication No. 11-54934).
The above manufacturing method employs an approach that as a generally used interlayer electric connecting system, a through-hole penetrating all layers is formed and each of the layers is electrically connected by plating the through-hole. However, in this electric connection system, a processing method is simple, but there are so many restrictions in circuit designing. The worst disadvantage is that since all the layers are electrically connected through the penetrating through-hole plating, the outermost layer may have through-hole plating connecting lands, which thus occupy a larger area. This makes it difficult to increase a circuit-mounting density, which may be a critical problem in component mounting or circuit wiring. Furthermore, it leads to a system incapable of achieving high-density mounting or high-density wiring which will be increasingly needed in the market.
For inexpensive manufacturing, a flexible printed circuit board is prepared using a multiple pattern where a plurality of patterns are arrayed in one sheet. Thus, a multilayered flexible printed circuit board can be inexpensively prepared through a similar manufacturing process. However, if a defective patterning is present in a sheet in this manufacturing process, a multilayered flexible printed circuit board formed by stacking the defective patterning sections is defective, leading to reduction of a process yield in a stacking step.
The biggest difference between a multilayered flexible printed circuit board or rigid-flex printed circuit board and a multilayered rigid printed circuit board is the presence of a flexible section. Preparation of such a flexible section requires removal of an outer layer for preventing stack of the flexible section or removal of an outer layer after stack, leading to reduction of the circuit board number per sheet in sheet stack. Furthermore, in a pattern design in which individual layers have different sizes, the circuit board number per one sheet is restricted to the least circuit board number of the circuit board numbers in the individual layers, leading to reduction of the circuit board number per sheet.
The latter manufacturing process has a special step of forming a hole in a receiving substrate in a conductor post by laser processing, desmearing it and forming a surface-coated opening, and establishing these techniques and improving their yields are also problems. Furthermore, as the layer number is increased, a production time and a cost is increased and a material cost for surface coating is also increased.
PATENT DOCUMENT 1: Japanese Laid-open Patent Publication No. 11-54934